Why don't you limit the amount of unmapped pages for the whole system?
Current implementation, which limit unmapped pages per zone, may cause unnecessary
reclaiming. Because if memory access is not balanced among zones(or nodes),
the kernel may reclaim unmapped pages even though other zones/nodes have enough
spaces for them.

Anyway, I'm interested in this patchset. Because my customers in enterprise area
want this kind of feature for a long time to avoid direct reclaim completely
in a certain situation.